1. Field of the Invention
The present invention relates to a power supply apparatus which generates DC power supply voltage, and to an insulation DC converter using a transformer, and more particularly, to a synchronous rectification type switching power supply apparatus which carries out rectification on a secondary side by synchronous rectification control.
2. Description of the Related Art
As a circuit which converts input voltage using a transformer and outputs DC voltage of different potential, there is an insulation DC converter as shown in FIG. 7. This converter is called a half bridge type converter. The converter uses diodes D1 and D2 for rectifying current flowing from a secondary coil, and alternately charging a smoothing capacitor Co with current rectified by the diodes, thereby producing DC voltage. A circuit structure of such a diode bridge type rectifier circuit is simple, but there is a problem that rectification loss Vf·I is generated by forward voltage Vf of the diodes and current I flowing through the diodes.
Hence, as shown in FIG. 8, there is proposed an invention concerning a synchronous rectification type DC converter in which the diodes are replaced by MOSFET SW1 and SW2 having small on resistance, the MOSFET is on/off controlled by synchronous control to carry out the rectification (e.g., Japanese Patent Application Laid-open Publication No. 2001-292571).
In the synchronous rectification type converter described in the above-mentioned Japanese Patent Application Laid-open Publication No. 2001-292571, a sense resistor Rs is connected to switching MOSFET SW1 and SW2 in series, on/off timing of the SW1 and SW2 is detected by voltage reduction of the resistor, thereby generating a control signal. Therefore, the losses at the MOSFET SW1 and SW2 are small, but a loss is generated by the sense resistor Rs. In order to reduce this loss to a negligible value, it is necessary to reduce the resistance value, but there is a problem that if the resistance is reduced, voltage drop becomes small, and therefore, a precise and complicated detection circuit (comparator) is required.
To reduce the loss, the present inventors have invented a method for producing on/off control signals of SW1 and SW2 by detecting voltage between source and drain of the switching MOSFET SW1 and SW2 instead of connecting the sense resistor Rs. According to this method, however, there is a problem that since the voltage reduction caused by resistance component (usually a few mΩ) of the MOSFET having only on resistance having negligible loss is extremely small, there is a problem that a precise and stable detection circuit (comparator) having small variation is required.